This invention relates to an apparatus for image sensing of three-dimensional structures for automatic inspection and other applications.
In a known imaging system, matrix cameras (i.e. areascan cameras) are used based on sensors such as a charge-coupled device (CCD) using a two-dimensional array of sensing elements. Matrix cameras are widely used in video cameras, closed circuit TV cameras (CCTV), and camcorders, and may be used to capture images of three-dimensional structures.
A problem with using a matrix camera is that only part of the three dimensional structure will be visible to the camera. For example, when imaging the surface of a cylinder or a sphere, the camera will only see the surface nearest the camera and will not be able to see the sides or back surfaces. This means that a multiple number of images will be needed to build up a complete all round image of the structure. In a practical application such as automatic inspection system, this is a disadvantage since capturing and processing multiple images imposes a heavier processing load, hence impacting system cost, than would be the case for a single image.
A second problem with using a matrix camera is that any non-flat areas of the structure will be projected onto the sensor in a distorted manner. For example, the walls of a cylindrical or spherical structure will produce distortion of the image as the surfaces curve away from the camera. This means that the image processing system must correct for this distortion when inspecting images containing surface detail, for example printed characters on the surface. This type of correction means significantly increased complexity and hence increased cost for the image processing system.
A third problem with using a matrix camera is that it becomes necessary to tile together multiple images. This applies where the surface being imaged contains patterns which may straddle two or more of the multiple images and it becomes necessary to tile (i.e. splice together) these images to reconstruct the complete image. This results in significant additional complexity in the image processing system and introduces the risk that spurious xe2x80x9csplicing artefactsxe2x80x9d may be created in the reconstructed image.
In another known image sensing system, a linescan camera is used to capture an image of a three dimensional structure. The linescan camera is arranged to form an image of a long narrow portion of the structure. After a suitable integration time which allows the image to be built up on the linescan sensor, the line image is read out of the camera in the form of a line of image pixels (i.e. picture elements) and transferred to an image storage and image processing system. The structure is arranged to move relative to the camera so that the process can be repeated on an adjacent long narrow portion of the structure, and eventually through a multiplicity of portions, a two-dimensional array of pixels is obtained.
A typical example of linescan imaging would be forming an image of a cylindrical surface whereby the cylindrical structure is arranged to rotate about its principle axis whilst a linescan camera captures a series of line images along the cylinder wall in direction parallel to the major axis.
A problem with linescan imaging is that it is optically inefficient. The camera""s lens is capable of imaging an area wider than a narrow portion of the structure and illumination systems will also illuminate a wider portion of the structure. The linescan camera uses only a small part of the available image and discards the rest. This optical inefficiency leads to limitations in the overall imaging system, limiting the speed of image capture, and demanding added complexity of high intensity illumination.
A second problem with linescan imaging is image smearing (i.e. image blur). In a typical practical system, the structure is arranged to move at a constant speed relative to the camera so that successive lines of pixels are obtained at regular physical displacements around the structure. This means that any feature on the surface of the structure is moving relative to the camera and will tend to blur in the image to the extent of the integration time used by the camera. This will be most critical with fine detail on the surface of the structure, such as small dots or lines, whose size is similar to, or 1-5 times larger than, the size of the pixels being imaged at the structure. The overall effect of image smearing is that the quality of the captured image will be reduced with a loss of contrast and loss of image sharpness particularly affecting fine detail such as dots and lines.
In a known variant of linescan camerasxe2x80x94time delay integration (TDI) camerasxe2x80x94some of the problems of linescan imaging are overcome. In a TDI linescan camera, multiple parallel lines of pixels are imaged simultaneously. This means that the width of the imaged area is increased, for example to 8, 16, 32 or 96 parallel lines of pixels, depending on the particular imaging device used. In a TDI system, a shift register method is used to shift the image being integrated on the sensor such that the partially integrated image on the sensor tracks the movement of the structure. Hence each pixel in the read out will have been exposed for 8, 16, 32, or 96 clock periods. This increases the optical efficiency of the system.
A problem with TDI imaging is that image smear is still present for the same reasons as a basic linescan camera, leading to a loss of image sharpness and contrast on fine detail. A second problem with TDI cameras is their relatively high costs due to their specialised uses and consequent low volumes of manufacture.
A further problem with both normal linescan cameras and TDI linescan cameras is that imaging is restricted to applications where the camera can be focused on a line along the three dimensional structure. Given practical considerations of standard lenses and depth of field (for maintaining the image adequate sharpness of image), this mean that linescan systems are best suited to flat walled structures such as cylinders and are not well suited to more complex surfaces, for example, spherical structures.
According to the present invention, there is provided an apparatus for providing a two-dimensional representation of the surface of a three-dimensional object comprising means for translating the object along a path, and means for simultaneously rotating the object about at least one of its axes, means for sensing the two-dimensional representation, means for imaging a portion of the object surface onto a portion of the sensing means, the imaging means being translatable along a path parallel to the object path, the rates of translation of the object translating means and the imaging means, and of rotation of the object are selected so that the combination of the rotational and translational movement of the object and imaging means causes successive images of adjacent portions of the object surface to be imaged on successive portions of the sensing means, as the object travels along a portion of the object path, thereby capturing a two dimensional image of the surface of the object.